GEOLOGY | April 2014 | www.gsapubs.org 1

ABSTRACTIn the modern silica cycle, dissolved silica is removed from sea-

water by the synthesis and sedimentation of silica biominerals, with additional sinks as authigenic phyllosilicates and silica cements. Fundamental questions remain, however, about the nature of the ancient silica cycle prior to the appearance of biologically mediated silica removal in Neoproterozoic time. The abundance of siliceous sedimentary rocks in Archean sequences, mainly in the form of chert, strongly indicates that abiotic silica precipitation played a signifi -cant role during Archean time. It was previously hypothesized that these cherts formed as primary marine precipitates, but substantive evidence supporting a specifi c mode of sedimentation was not pro-vided. We present sedimentologic, petrographic, and geochemical evidence that some and perhaps many Archean cherts were deposited predominately as primary silica grains, here termed silica granules, that precipitated within marine waters. This mode of silica deposi-tion appears to be unique to Archean time and provides evidence that primary silica precipitation was an important process in Archean oceans. Understanding this mechanism promises new insights into the Archean silica cycle, including chert petrogenesis, microfossil preser-vation potential, and Archean alkalinity budgets and silicate weather-ing feedback processes.

INTRODUCTIONIt has been suggested that primary chemical precipitation of amor-

phous silica played a major role as a silica sink during Precambrian time (Lowe, 1999a; Maliva et al., 2005; Posth et al., 2008; Siever, 1992), al-though unambiguous examples of primary silica phases were elusive. Pre–3.0 Ga Archean sedimentary units include abundant chert litholo-gies formed through silica replacement and/or cementation of volcanic ash, detrital sediments, and a variety of other primary sediment types. One common element of these cherty sequences is the occurrence of lay-ers or bands of white- to light gray–weathering chert, often translucent, generally <10 cm thick, and composed of nearly pure SiO2 (>99 wt%) (Lowe, 1999a). These layers are widely interbedded with carbonaceous layers containing trace organic matter (Lowe, 1999a; Walsh and Lowe, 1999), ferruginous bands, or sideritic layers of comparable thickness to form black and white banded chert, banded iron formation, and banded ferruginous chert, respectively.

Banded black and white cherts have been considered likely candi-dates for primary silica precipitates; early deformation features (Lowe, 1999a) and oxygen isotopic data (Hren et al., 2009; Knauth and Lowe, 1978, 2003) are consistent with primary or earliest diagenetic band for-mation. Two hypothetical band formation mechanisms have been pro-posed: (1) primary precipitation of silica on the seafl oor (Lowe, 1999a; van den Boorn et al., 2007), and (2) earliest diagenetic segregation of adsorbed silica, originally deposited homogeneously with carbonaceous matter and/or iron oxides, into distinct layers (Lowe, 1999a). While many white chert layers are massive, a surprising number display pre-served internal granular textures characterized by sand-sized grains of nearly pure silica. This observation suggests that many, if not all, white chert bands originated via a third novel mechanism, i.e., deposition of primary silica grains. These pure to nearly pure silica particles are here termed silica granules.

SAMPLES AND METHODSOutcrops, polished hand samples, and polished petrographic thin

sections were used to examine silica granules. Some samples are from the BARB4 core from the 2011 International Continental Scientifi c Drilling Program drilling project in the Barberton Greenstone Belt (South Africa). Elements of interest (Ca, Mg, Fe, Al, and P or Ti) were mapped in carbon-coated (~14 nm thick) polished thin sections using a JEOL JXA-8200 advanced electron probe microanalyzer at the Division of Geological and Planetary Sciences Analytical Facility at the California Institute of Technology (Pasadena, California, USA) and using the JEOL JXA-8230 SuperProbe electron probe microanalyzer at the School of Earth Sciences Mineral Analysis Facility at Stanford University (Stanford, California). Qualitative intensity maps without background corrections were col-lected, operating the electron probe in wavelength dispersive X-ray spec-trometer mode at 15 kV accelerating voltage, 100 nA beam current, and 100 ms dwell time.

OBSERVATIONSSilica granules are round, internally unstructured, sand-sized silica

particles (Fig. 1). The granules are composed of essentially pure micro-crystalline quartz, although minor Fe-bearing impurities, especially sid-erite and hematite, occur locally. The occurrence of these silica grains is not limited to white chert bands. In many cases, cherty layers as much as 50 cm thick are composed largely of silica-rich grains, and many detrital sedimentary deposits include virtually pure silica grains mixed with a vari-ety of carbonaceous, volcaniclastic, and other sediment and particle types. Most granules display evidence of compaction (Fig. 2); granule cross sec-tions are elliptical in planes perpendicular to the bedding plane, with an average grain shape of an oblate spheroid. Unlike boudins, barrel-shaped deformation structures formed during extension, compacted granule cross sections are similar along any plane perpendicular to the bedding plane. This compaction and the current microcrystalline nature of the granules exclude an origin as monocrystalline quartz sand, suggesting instead an initial composition as amorphous silica.

Granules can be easily distinguished in hand specimen and thin sec-tion when not compacted, but are more diffi cult to recognize when se-verely fl attened (Fig. 2) or when the surrounding cement is also composed of nearly pure and largely homogeneous microquartz (Figs. 1A and 2A). Some larger silica grains appear to be aggregates of individual granules (Fig. 1C), but the sand-sized granules are unstructured in thin section and distinct from other types of subspherical grains within the same Archean sequences, such as accretionary lapilli (Lowe, 1999b) or impact spherules (Lowe and Byerly, 1986). They lack relict textures that would indicate diagenetic transformation or replacement of primary carbonate (Maliva et al., 2005) or volcanic (DiMarco and Lowe, 1989) grains. Electron probe maps of Al, Fe, Ca, Mg, P, and Ti do not reveal internal structuring (Fig. 3) and support petrographic observations that granules are generally distin-guishable from the surrounding microquartz matrix because they contain few trace impurities.

Granules are common in sedimentary units representing a variety of depositional environments, from intertidal to deep-water basinal settings in the Barberton Greenstone Belt in South Africa and the Pilbara Craton in Western Australia. In the following discussion we describe the proper-ties and distribution of granules in four specifi c occurrences that provide examples of the morphology and environmental diversity of granules in the 3.2–3.5 Ga Barberton and Pilbara sequences.*E-mail: ltrower@stanford.edu.

Primary silica granules—A new mode of Paleoarchean sedimentationElizabeth J.T. Stefurak1*, Donald R. Lowe1, Danielle Zentner1, and Woodward W. Fischer2

1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91126, USA

GEOLOGY, April 2014; v. 42; no. 4; p. 1–4; Data Repository item 2014106 | doi:10.1130/G35187.1 | Published online XX Month 2013

© 2014 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.

2 www.gsapubs.org | April 2014 | GEOLOGY

A

B

C

1 mm1 cm

BA

0

10

20

30

40

50C

500 μm500 μm500 μm rela

tive

inte

nsity

- Fe

WD

S

A B

C

G

E

D

QS

SS

Q

F

Figure 1. Silica granules. Granules are outlined (dotted lines) for clar-ity in right panels of A–C. Scale bars are 500 µm. See Table DR2 (see footnote 1) for more detailed stratigraphic information for samples shown. A: Pure silica granules from within white chert band, barely distinguishable due to presence of trace carbonate inclusions in sur-rounding material. Upper Mendon Formation, Onverwacht Group, South Africa. B: Granules (possibly including aggregates of granules, based on irregular shapes) with carbonaceous grains and matrix. Lower Mapepe Formation, Fig Tree Group, South Africa. C: Large in-traclast (dashed outline) composed of round, uncompacted granules. Upper Mendon Formation. D: Granules rimmed with diagenetic hema-tite (upper left) or completely fi lled with diagenetic siderite and hema-tite (lower right). Antarctic Creek Member at base of Mount Ada Basalt, Warrawoona Group, Western Australia. E, F: Pure silica granules in plane-polarized light (E) and cross-polarized light (F), some rimmed with diagenetic siderite (S) or replaced with coarse quartz cement (Q). Antarctic Creek Member at base of Mount Ada Basalt. G: Lens of silica granules within ferruginous shale, including micron-scale hematite grains within matrix. Lower Mapepe Formation.

Figure 2. Hypothetical sequence of progressive com-paction of silica granules in white chert band illustrates how matrix material can take on appearance of anas-tomosing laminations with increasing compaction. A: Minimally compacted. B: Moderately compacted. C: Severely compacted. Scale bars are 500 µm. See Ta-ble DR2 (see footnote 1) for more detailed stratigraphic and location information for samples shown.

Figure 3. Layer of silica granules within banded iron formation in Mapepe Formation, South Africa. A: Hand-sample scale. Granule layer indicated by dashed outline. B: Magnifi ed view showing grains visible on sample surface. C: Representative electron microprobe relative intensity map of Fe (WDS—wavelength dispersive X-ray spectrometer). Note that granules are distinguishable by their rela-tive lack of trace minerals. Main Fe-bearing phases in this example are siderite (larger, euhedral grains) and hematite (smaller, micron-scale grains). See Table DR2 (see footnote 1) for more detailed strati-graphic information for sample shown; see Figure DR1 for additional electron microprobe maps from this sample.

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Within the largely basaltic Warrawoona Group (Pilbara Craton), the Antarctic Creek Member is a 4–14-m-thick unit that includes silicifi ed felsic volcaniclastic sediments and a spherule bed formed by meteorite impacts correlated with a similar 3472 Ma unit in the Barberton belt (Glik-son et al., 2004; Hickman and Van Kranendonk, 2008; Lowe and Byerly, 1986). The Antarctic Creek Member includes many beds composed large-ly of current-worked, subspherical, sand-sized silica granules. The gran-ules occur as the main components of lenticular, 10–30-cm-thick, grain-supported layers that locally display dune cross-bedding. Some granules are composed purely of microquartz, while others contain trace amounts of fi nely disseminated iron oxides and carbonates (Figs. 1D–1F). Most granules are oblong in cross section, with aspect ratios from 1:1 to 4:1. The granules are cemented by a combination of microquartz and coarse quartz, with admixed hematite or goethite in the more reddish layers. We interpret these cross-stratifi ed granule layers as current-deposited bars or shoals in a shallow subtidal to intertidal paleoenvironment; they demon-strate that silica granules were deposited in energetic shallow-water set-tings, could be transported as sand-sized debris, and were suffi ciently re-sistant that they could be swept into dunes and other bedforms by currents.

Examples from the Barberton Greenstone Belt suggest that granules are not limited to shallow-water settings. We consider granules in three stratigraphic units: the ca. 3400 Ma Buck Reef Chert, the ca. 3300 Ma Mendon Formation, and the ca. 3260 Ma Mapepe Formation. The Buck Reef Chert is a 200–400-m-thick unit of banded black and white and banded ferruginous cherts at the base of the Kromberg Formation (Lowe and Byerly, 1999), and represents depositional environments ranging from a shallow, restricted evaporitic setting at the base (Lowe and Fisher Wor-rell, 1999), to a relatively open shelf toward the middle, to a deeper, qui-eter setting marked by fi nely laminated, banded ferruginous cherts toward the top (Lowe and Byerly, 1999). The uppermost Mendon Formation is an ~50-m-thick unit of black chert, banded black and white chert, and banded ferruginous chert deposited in relatively quiet water below storm wave base (Lowe and Byerly, 1999). The basal Mapepe Formation, which rep-resents a quiet and probably deep-water setting (Lowe and Nocita, 1999), includes thick units of fi ne ferruginous shale, banded ferruginous chert, and, in a few areas, hematitic banded iron formation defi ned on a lamina-tion scale by alternations of sideritic, hematitic, and cherty layers.

There are three types of occurrences of silica granules within black cherts of the Buck Reef Chert and the upper Mendon Formation: (1) ma-trix-supported granules within a microquartz groundmass that contains trace inclusions of carbonaceous matter, phyllosilicates, and/or carbonate phases (dolomite and ankerite) (Figs. 1A and 2A–2C); (2) grain-support-ed, current-deposited granules mixed with carbonaceous and/or volcanic grains (Fig. 1B); and (3) larger, irregularly shaped grains and intraclasts, many of which appear to have formed as aggregates of sand-sized gran-ules, mostly occurring in comparatively high energy event beds mixed with other intraclasts in addition to carbonaceous and volcanic grains (Fig. 1C). Granules are often strongly compacted, with aspect ratios in excess of 10:1 (e.g., Fig. 2C).

There are two types of occurrences of silica granules within the ferru-ginous lithologies of the Mapepe Formation: (1) as centimeter-scale, len-ticular layers interbedded with ferruginous shale (Fig. 1G), and (2) as sub-centimeter-scale lenticular layers within banded iron formations (Fig. 3). The granule layers in the banded iron formations are less abundant than other layer types and stand out by their lenticularity, suggesting that gran-ule deposition was locally superimposed on slow background accumula-tion of material settling out of suspension (Fischer and Knoll, 2009). Some granules are composed purely of microquartz, while others contain trace inclusions of hematite that may represent oxidized siderite. Granules in this setting display a range of compaction effects, with maximum aspect ratios of ~5:1 (Fig. 3C). Grain-supported layers are typically silica cemented, whereas matrix-supported layers contain micron-scale hematite and sider-ite grains (in addition to microquartz) in the matrix.

PROCESSES OF GRANULE FORMATION AND DEPOSITIONAll granules examined to date are sand-sized subspherical grains that

show evidence of compaction and lack internal structure. These character-istics suggest a common origin and provide several clues to the processes of granule formation and deposition.

Silica granules are subrounded to rounded and subspherical in shape; this could be a result of primary precipitation mechanism, abrasion of ini-tially more angular and irregular grains during transport, or some com-bination of both. If the granules had originated as abraded rip-up clasts from a bed of amorphous silica precipitated on the seafl oor, analogous to muddy intraclasts, we might expect at least rare occurrences of granules with irregular shapes or internal layers or lamination. However, the gran-ules are consistently subspherical, have rounded grain shapes, and lack in-ternal structures. These characteristics are most consistent with formation in suspension, where precipitation could occur radially at subequal rates.

Granules display cross-sectional aspect ratios from 1:1 to >10:1, representing a range of compaction and cementation timing. The least compacted granules must have been cemented rapidly during earliest diagenesis, implying that, under some conditions, seawater or pore fl uids could promote rapid precipitation of silica. Minimally compacted gran-ules could therefore prove useful indicators for silicifi cation conditions with relatively high microfossil preservation potential.

There is no apparent relationship between granule size and deposi-tional water depth (Table DR1 in the GSA Data Repository1). This implies that precipitation was limited to a specifi c depth zone within the water column, because granule size might otherwise be predicted to increase with depth as longer settling or transport time allowed more silica to precipitate. Archean seawater was likely near saturation with respect to amorphous silica (Siever, 1992) and silica solubility would have decreased with water depth; silica is signifi cantly less soluble at lower temperatures (Siever, 1962) and is not particularly sensitive to pressure (Willey, 1974). Theoretical rates of aggregation or coagulation of silica via polymeriza-tion would be highest at low temperatures (<60 °C) and either (1) slightly acidic pH (pH range of 4–7), or (2) at higher pH (pH range of 7–10) in the presence of electrolytes (Iler, 1979; Williams and Crerar, 1985). Either of these conditions might be met in shallow-water environments, which could have had slightly acidic pH due to interaction with a CO2-bearing atmosphere (Hessler et al., 2004; Kasting, 1987) or high salinity (Knauth, 2005) due to evaporative concentration. The observation of silica granules in a variety of depositional water depths, including intertidal settings, also supports granule formation in relatively shallow water.

CONCLUSIONSThe widespread occurrence and abundance of silica granules in these

pre–3.2 Ga sequences indicate that they represented a signifi cant silica depositional mode during Paleoarchean time. It is also very likely that many massive, structureless silica layers and bands in these sequences represent accumulations of granules lacking impurities that would allow petrographic or geochemical discrimination of granules and the surround-ing matrix and/or cements. Although the silica granules described here bear some resemblance to granular iron formation (Simonson, 2003), as chemical grains particular to Precambrian time, their unique mineralogy and occurrence in nonferruginous settings suggest that silica granules formed by mechanisms and/or conditions different from those of granular iron formation. Granule formation and deposition appear to have repre-sented a substantial sink for dissolved silica in Paleoarchean oceans, with additional sinks of banded iron formations, authigenic phyllosilicates,

1GSA Data Repository item 2014106, Figure DR1 (additional electron probe maps of area shown in Fig. 3C), Table DR1 (granule size data), and Table DR2 (stratigraphic information for samples shown in fi gures), is available online at www.geosociety.org/pubs/ft2014.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

4 www.gsapubs.org | April 2014 | GEOLOGY

and silica cements. Because the formation of clays via reverse weathering consumes alkalinity and short-circuits the silicate weathering feedback (Michalopoulos and Aller, 1995), understanding the contributions of each silica sink has important implications for climate, seawater chemistry, and the carbon cycle during Archean time. Silica granules are notably absent from younger rocks, indicating that the dominant mode of silica deposi-tion has evolved over geologic time, possibly driven by secular changes in continental growth, weathering, ocean composition, and the biological silica cycle.

ACKNOWLEDGMENTSWe thank C. Ma (electron probe, Caltech) and R. Jones (electron probe, Stan-

ford) for their assistance. Stefurak was supported by a National Science Founda-tion graduate fellowship. Fischer was supported by the National Aeronautics and Space Administration Exobiology program (grant NNX09AM91G) and the David and Lucile Packard Foundation. The School of Earth Sciences, Stanford University, provided funds to Lowe. We are grateful to Sappi Forest Products, the Mpumalanga Parks Board (J. Eksteen and L. Loocks), and Taurus Estates (C. Wille) for access to private properties.

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Manuscript received 9 October 2013Revised manuscript received 18 December 2013Manuscript accepted 20 December 2013

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